This invention relates generally to a method for preventing antifoulant in light end hydrocarbons or blends of crude oil and light in hydrocarbons.
All crude oils are composed of two major components, a low molecular weight oil fraction (aromatic and/or saturates), and a high molecular weight fraction insoluble in paraffinic solvents. This fraction is called C.sub.7 -asphaltenes. As used herein the term "asphaltenes" refers to these high molecular weight paraffinic insoluble asphaltenes. Asphaltenes are characterized by a high average molecular weight (about 1000 and up to 5,000) and very broad molecular weight distribution (up to 10,000) and high coking tendency. Asphaltenes fouling refers to the C.sub.7 -asphaltenes precipitating out in pipelines, storage vessels, transport vessels and especially in process equipment (e.g. heat exchangers, reboilers, etc.) operating at elevated temperatures.
Highly paraffinic hydrocarbon liquids (those with no aromatics) generally do not present fouling problems in and of themselves because of the absence of asphaltenes. However, it frequently is desirable, if not necessary, to combine the paraffinic liquids such as ethane, propane, LPG's, condensates, etc. with crude oil for purposes of transporting or storing the paraffinic hydrocarbon liquid. The addition to crude oil, particularly crude low in aromatics, frequently results in equipment fouling during transport and storage and subsequent processing as when the paraffinic hydrocarbon liquid is separated from the crude oil as by distillation separation. The separation operation is normally carried out at temperatures ranging from 100.degree. to 400.degree. F., more usually at 200.degree. F. to 400.degree. F. The presence of even small quantities of asphaltenes in the blend causes fouling in the heat exchangers and distillation equipment (e.g. reboilers).
It has recently been discovered that certain light end hydrocarbons, such as liquified natural gas, present asphaltene fouling tendencies with minute quantities of high molecular weight asphaltenes present (from 10 to 10,000 ppm). The liquid hydrocarbon gases may also be subjected to fraction separation processes at elevated temperatures (100.degree. F.-400.degree. F.) which appears to accelerate asphaltene fouling.
The present invention relates to the use of overbased magnesium sulfonate as an asphaltene antifoulant for crude oil/paraffinic hydrocarbon liquid blends and liquified hydrocarbons. The term liquified petroleum gases include liquified petroleum gas (LPG), natural gas liquid (NGL), as well as C.sub.2 -C.sub.7 alkanes such as ethane, propane, butane, etc.
U.S. Pat. No. 3,328,283 discloses the use of organic sulfonic acid or alkyl aryl sulfonate salt in combination with a partial ester. U.S. Pat. No. 3,328,284 discloses the use of an alkyl aryl sulfonate with an oxy alkylated phenolic compound. In both of these prior art patents, the sulfonate is not overbased. Moreover, the alkyl aryl sulfonates appears to function as an adjunct to the main antifouling agent, and not as an antifoulant per se.
Overbased alkyl aryl sulfonates have been used as antifoulants for heat transfer oils. Heat transfer oils, however, are refined oils that do not contain asphaltenes and therefore are not particularly relevant to the present invention which is directed specifically at mitigating asphaltene fouling. The transfer oil patents of the prior art include U.S. Pat. Nos. 3,554,914, 3,920,572, and 3,958,624.